The present invention relates to a method and a system of reading a radiation image that has been stored in a photostimulable phosphor screen. More particularly the invention relates to the re-use of the photostimulable phosphor screen.
Radiation image recording systems wherein a radiation image is recorded on a photostimulable phosphor screen by exposing said screen to image-wise modulated penetrating radiation are widely used nowadays.
The recorded image is reproduced by stimulating the exposed photostimulable phosphor screen by means of stimulating radiation and by detecting the light that is emitted by the phosphor screen upon stimulation and converting the detected light into an electrical signal representation of the radiation image.
In such a system it is preferred, in view of economy, that the stimulable phosphor screen can be used in many imaging cycles.
The reuse of the stimulable phosphor screen is possible when the previously stored radiation image is erased to a sufficient extent.
When reading out an image by stimulating a phosphor screen that has been exposed to penetrating radiation, less than 90% of the stored energy is released. Thus there arises a problem that, upon reuse, part of the radiation image is still stored in the phosphor screen and can appear in the subsequent image as a so-called ghost image.
In general medical radiography, images are made with widely differing X-ray doses.
To make images of extremities, like e.g. fingers, doses are used of the order of 1 mR. On the other hand, images of internal organs, like the stomach are made with X-ray doses that may be as high as 300 mR.
To avoid ghosting, when making a 1 mR image immediately after a 300 mR image, the signal of the first image must be reduced by more than a factor of 300.
As a matter of fact, a dynamic range is desired in the second image of at least 100. This implies that the signal created by the first irradiation must be reduced by a factor of at least 3.104, which is equivalent to requiring an erasure depth of 1/(3.104)=3.3.10xe2x88x925.
According to U.S. Pat. No. 3,859,527 (column 4, lines 5-7) the phosphor can be reduced to neutral state by actions like a uniform illumination, irradiation or heating.
In commercial systems, the phosphor screen is erased by illumination with visible light. Incandescent lamps are commonly used because they are cheap, high power light sources.
High power is needed, because in order to guarantee a high through-put scanning system, the phosphor screen must be erased in a short time.
High power lamps, however, generate a lot of heat, which may destabilise the scanner to read out the storage phosphor screens. The size of the read out apparatus has to be rather large in order to enable removal of the heat generated by a powerful erasure unit which is required in order to have a high throughput.
In order to develop a reliable and compact storage phosphor screen digital radiography system it is important to reduce the power consumed by the erasure unit. This was not possible, however, without affecting the system""s through-put in a negative way.
It is an object of the present invention to provide a method and a system for reading a radiation image that has been stored in a photostimulable phosphor screen wherein the screen is erased in between successive recordings to an adequate extent so as to permit re-use of the screen.
It is a further object of the present invention to provide such a system that is compact and has at the same time a high throughput.
Further objects will become apparent from the description given below.
The above mentioned objects are realised by a method of reading a radiation image that has been stored in a photostimulable phosphor screen having a surface area that is not greater than Smax comprising the steps of
(1) stimulating said phosphor screen by means of stimulating radiation,
(2) detecting light emitted by the phosphor screen upon stimulation and converting the detected light into a signal representation of said radiation image,
(3) erasing said phosphor screen by exposing it to erasing light, characterised in that
(4) said photostimulable phosphor screen comprises a divalent europium activated cesium halide phosphor wherein said halide is at least one of chloride and bromide and said erasing light is emitted by an erasing light source assembly emitting in the wavelength range of 300 nm to 1500 nm and having an electrical power not greater than Smaxxc3x971 J.
The terms xe2x80x98an erasing light source assemblyxe2x80x99 refer to either a single light source or a group of more than one erasing light source. In the latter case the electrical power which is specified is the electrical power of the total assembly.
According to the invention the wavelength of the erasing light source(s) is within the range of 300 nm to 1500 nm. A wavelength range from 500 nm to 800 nm is preferred because a divalent europium activated cesium halide phosphor is most efficiently erased with light within this wavelength range.
Another aspect of the present invention relates to a radiation image read out apparatus for reading a radiation image that has been stored in photostimulable phosphor screen having a surface area not greater than Smax, having
a source of stimulating radiation arranged for emitting stimulating light and directing said light onto a photostimulable phosphor screen,
a transducer for converting light emitted by said phosphor screen upon stimulation into an electrical signal,
an erasing unit for erasing said photostimulable phosphor screen after having been stimulated, characterised in that
said photostimulable phosphor screen comprises a divalent europium activated cesium halide phosphor wherein said halide is at least one of chloride and bromide, and
said erasing unit comprising at least one erasing light source, the total electrical power of the erasing light source(s) of the erasing unit being smaller than Smaxxc3x971 J.
Still another aspect relates to a re-usable radiation detector comprising
a photostimulable phosphor screen,
at least one source of stimulating light arranged for stimulating said phosphor screen,
an array of transducer elements arranged for capturing light emitted by the phoshor screen upon stimulation and for converting said light into an electrical signal representation,
an erasing unit comprising at least one erasing light source,
means for transporting the phosphor screen and an assembly of stimulating light source(s), said array of transducer elements and said erasing unit relative to the each other,
an enclosure enclosing said photostimulable phosphor screen, said assembly of stimulating light source, erasing light source, said array of transducer elements, said transporting means,
interfacing means for communicating said electrical signal representation to an external signal processing device.
Specific features for preferred embodiments of the invention are disclosed in the dependent claims.
The objects of the present invention are realised in a read out method and apparatus and detector according to the present invention by using a phosphor with a very good erasibility combined with an erasure unit with low electrical power.
The inventors have found that the use of an erasing light source of a low power type is adequate in a system wherein a radiation image is temporarily stored in a divalent europium activated cesium halide phosphor without there being any need for the phosphor screen to remain for an extended period of time in the erasing unit. Consequentially the fact that a low power erasing light source is used has no negative influence on the throughput of the system.
Furthermore, since a low power erasing light source has a low heat dissipation, no particular precautions (such as the provision of fans or the like) have to be taken to dissipate the heat generated by the erasing light source. Consequentially the read out system can be made very compact and still remain very reliable.
In this document the term xe2x80x9cradiationxe2x80x9d has to be understood as any penetrating radiation and includes irradiation originating from a radioisotope (e.g. Co60, Ir192, Se75, etc.), radiation created by an X-ray generator of any type, radiation and high energy particles created by a high energy radiation generator (e.g. Betatron), radiation from a sample labeled with a radioisotope as is the case in e.g. autoradiography.
The present invention as well as specific and/or preferred embodiments hereof will be explained in the detailed description given below. Particular aspects will be illustrated by the drawings enumerated hereinafter.